1. FIELD OF THE INVENTION
The present invention is a continuation-in-part of application Ser. No. 648,330, filed Jan. 30, 1991, now U.S. Pat. No. 5,109,694, in turn, a continuation of application Ser. No. 381,422, filed Jul. 18, 1989, now abandoned, and is directed to methods for estimating the vehicle body speed, on which a slip rate is based, for an anti-lock control system for a vehicle. The anti-lock control system operates by controlling the braking pressure for the wheels by estimating a vehicle body speed based on a wheel rotational speed detected in a speed sensor and by determining the slip rate of the wheels based on the estimated vehicle body speed.
2. DESCRIPTION OF THE PRIOR ART
Conventionally, anti-lock control of an object wheel is effected by estimating a vehicle body speed from a rotational speed of the object wheel and by determining a slip rate from a comparison of the rotational speed of the object wheel with a reference wheel rotational speed determined by the estimated vehicle body speed. Such a conventional system has been disclosed in Japanese Laid-open Patent Application Nos. 41237/82 and 60984/82.
During braking in a vehicle provided with an anti-lock control system that is travelling on a straight road, a wheel rotational speed which is the highest speed of the four front and rear wheels is a value closest to a vehicle body speed, during both anti-lock control and non-antilock control. During braking when the vehicle is turning, however, a large speed difference is produced between the highest one of the four-wheel rotational speeds and the other wheel rotational speeds due to a difference in speed between inner and outer wheels or between left and right wheels. Hence, if the vehicle body speed is estimated on the basis of the highest speed, without taking into account the turning action of the vehicle the reference wheel rotational speed determined from the estimated vehicle body speed to set the slip rate is higher than necessary. As a result, anti-lock control is carried out even though such control is not required.